Systems, such as computers, use fans to air cool individual components of the systems. Typically, a fan has to spin at a certain minimum speed to provide sufficient cooling to the components of a system. However, sometimes fans fail completely. At other times, they spin too slowly to prevent damage to the system's components from overheating.
A typical fan for use in electronic systems is an electronically commutated (i.e. brushless) DC (direct current) fan. The fan includes a DC motor with windings through which fan current flows. An integrated circuit commutates the fan current. Some manufacturers provide a TIL (transistor-transistor logic) pulse for every revolution of the fan. Such a logic pulse provides the capability of generating a tachometer function that indicates the fan's number of revolutions, for example, per minute. However, fans are not always available even for a higher price with this special TTL logic pulse option.
Other manufacturers use thermistors to detect that fans are rotating too slowly. A thermistor typically is heated by a power resistor. The power resistor, in turn, is sufficiently cooled by a properly operating fan. If the power resistor heats up due to a fan malfunction, then the thermistor provides an indication of the fan's failure. However, using thermistors is energy inefficient and cumbersome.
Furthermore, all these methods require an additional wire lead to the fan assembly. In installations where a large number of fans are installed, these methods can significantly increase the amount of wiring to such fans.